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Instrumentation Design Temperature Measurement

Thermowell Outer Diameter Larger than Nozzle Inside Diameter?

A scary part on-site is when you try to install the Thermowell inside the Nozzle and you realize the Thermowell Outer Diameter (OD) larger than Nozzle Inside Diameter (ID)?.

Thermowell Outer Diameter larger than Nozzle Inside Diameter

Image Courtesy: Parker

This simple thing will lead to great delays since you can’t just insert some other thermowell as they go through W.F.C calculations, material selection, etc.

Nobody would want this to happen, so Let’s explore the precautions and recommended practices.

What factors are to be evaluated for ensuring the Thermowell gets properly inserted in Nozzle I.D

1. Nozzle Schedule

Even 1&1/2 inch pipe won’t have the same I.D all the time.

The schedule of the pipe needs to be checked.

As it is logical the Higher the schedule (like Std, XS, XXS etc) the smaller the I.D available.

Software’s like pipe pro can be used to get dimensions.

2. Cladding on Thermowell

Thermowell consumes a lot of material as compared to the diaphragm of transmitters etc.

So always it is not a cost-effective option to upgrade thermowell material to Alloy C-276 or other exotic material until it’s a dire necessity.

Another option sometimes used (depends on application) is to provide cladding on thermowell.

This also consumes little space and must be accounted for.

3. Clearance Space

This one important consideration that gets overlooked.

Handling changes at the initial stages of the project is very easy, but the very same changes at a later stage have a significant impact.

A classic example is Wake frequency calculation that caused the Thermowell base O.D to increase. (Maybe in the later stage process data got revised or WFC was incorrectly done etc)

Now if we have exact cut to cut the distance between nozzle I.D and Thermowell O.D then it is very difficult to accommodate such last-minute changes.

Few more examples are listed below:-

Thermal expansion at higher temperatures is an issue.

As indicated above maybe at a later stage cladding of thermowell is recommended and this will consume some space.

Also sometimes the Pipe spec has a certain schedule but for ease of welling, a different schedule is selected for vessel nozzle.

Hence it is preferred not to take the exact cut to cut the distance and keep some clearance.

Beware of illegal Ways

Polishing of thermowell to fit into the Nozzle:-

Sometimes on-site people polish the thermowell to reduce its O.D so that it fits in the nozzle.

This is an unsafe practice. As this risks the integrity of thermowell design and can lead to catastrophic failure.

Thanks for reading !!

I hope it has been of value to you.

PS: This is as per the best of my current understanding.

Do you face any problems with Thermowells? Share with us.

Author: Asad Shaikh

Profile: Linkedin

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Thermowell Outer Diameter Larger than Nozzle Inside Diameter?

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